Diet of vittatus () in the northern Adriatic Sea M. Kovacic

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M. Kovacic. Diet of Gobius vittatus (Gobiidae) in the northern Adriatic Sea. Vie et Milieu / Life & Environment, Observatoire Océanologique - Laboratoire Arago, 2007, pp.27-33. ￿hal-03234882￿

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D I E T OF GOBIUS VITTAT U S (GOBIIDAE) IN THE NORTHERN A D R I ATIC SEA

M. KOVA I Prirodoslovni muzej Rijeka, Lorenzov prolaz 1, HR-51000 Rijeka, Croatia [email protected]

GOBIUS VITTATUS A B S T R A C T. – The Goby Gobius vittatus is a carnivore and generalist which feeds on a wide DIET variety of prey items, particularly on polychaetes, gastropods, copepods, ostracods and SEASONAL CHANGES ONTOGENETIC SHIFT bivalves. The intensity of feeding was lowest in autumn and the diet spectrum was broadest in FEEDING SELECTIVITY s u m m e r. There were high seasonal differences in biomass and number of almost all major prey items. Smaller specimens fed mostly on meiofauna, and large fish preferred macrofauna. T h e breadth of diet increased with fish size.

INTRODUCTION Four microhabitats were recognised as possible source of food at positions were G. vittatus was collected: plankton, scyaphilic The striped goby, Gobius vittatus Vinciguerra, 1883 is phyton, photophilic phyton and psammon. The phyton samples a poorly known Mediterranean gobiid species. T h e were scraped from rocky surfaces (0.01 m2) into plastic bags species was considered rare (Tortonese 1975) or very rare with a solid frame. The psammon samples were collected from (Jardas 1985) due to the lack of data. Morphology and an area of 0.01 m2 to the depth of 2 cm by pulling plastic bags habitat of this species are known from only a few papers with a solid frame through the sand. Samples of plankton were published since species description, based on one or two obtained by draining 3 l containers with air from scuba diving collected specimens (Kova i 2004). Heymer & Zander regulator and filling them up with sea water from the water col- (1978) described habitat, diet and morphology on 21 umn 0-0.3 m above the bottom. Phyton and psammon were specimens from Banyuls-sur-Mer (France). The results fixed in 65% alcohol. Sea water from the containers was filtered on diet were restricted to frequency and abundance analy- through a 100 µm mesh and the samples collected on the net ses of food on 17 specimens collected during two summer were also fixed in 65% alcohol. months. Morphology, habitat preferences and diet from Total length (Lt) of all individuals was measured to the near- these papers, which are the only known data on biology est 0.1 mm (later grouped in 5 mm length classes) and wet mass and ecology of the striped goby, were summarized by weighed (W) to the nearest 0.001 g after blotting dry on Miller (1986). The aim of the present research was to pro- absorbent paper. The specimens were dissected under low vide data on diet of G. vittatus, including diet composi- tion, feeding selectivity, and seasonal and ontogenetic diet shifts, and to compare different methods for diet analysis.

MATERIALAND METHODS

Seven hundred and four specimens of G. vittatus w e r e obtained on three locations (Stara voda, O tro and Selce) in the Kvarner area of the Northern Adriatic Sea, from April 2001 to March 2002 (Fig. 1). All fish were collected between 8 and 20 m depth, using a hand net and anaesthetic quinaldine during S C U B A dives. Twenty specimens were collected at each loca- tion during one dive every month. In two attempts in January on the location O tro, only four specimens were collected due to bad weather and low temperature. Therefore, the total sample of 704 specimens was collected at three locations in twelve months. All specimens were killed by over- a n a e s t h e t i z a t i o n with quinaldine and fixed in 65% alcohol. Both specimens of G. v i t t a t u s and available food were collected during the same dive Fig. 1. – The Kvarner area, Croatia. Collecting sites: (1) Stara in August and September 2001 at each of the three locations. voda, (2) O tro and (3) Selce. 28 M. KOVA I power binocular microscope for the removal of gut. Guts were quencies (Sokal & Rohlf 1995). Two-ways ANOVA was used to dissected and their entire content sorted to relevant taxonomic assess seasonal and ontogenetic differences in IF. Total length units, which were then counted. Sorted prey items and unidenti- and season were considered to be fixed factors. Homogeneity of fiable residue were weighed wet to the nearest 10 µg (Ohaus variance was tested with Levene’s test, and normality was tested

AP250D) after blotting dry on absorbent paper. Assigned wet with Kolmogorov-Smirnov test. IF was log-transformed to satis- masses independent of the ’s size were used for very fy the A N O VA assumptions. The mean and the confidence lim- small (Halacaridae, Ostracoda < 1 mm, Copepoda < its are displayed as the antilogarithm of the mean and the confi- 1 mm), estimated as average mass from weighing of larger sam- dence limits of the log-transformed data (Sokal & Rohlf 1995). ple of specimens. Weight of the entire gut content (WG C) was Tu k e y ’s test was used for post-hoc comparisons after A N O VA . calculated as the sum of weight of all prey items and of weight Data analyses were carried out by the Excel 2002 and the SPSS of unidentifiable residue. The samples of available food were 9.0 program. kept in Rose Bengal (1 g in 1 l 65% alcohol) for 24 h. Plankton samples were then sorted to relevant taxonomic units, which were counted. Very light organisms of available food in samples RESULTS of sands and aufwuchs were extracted by a modified elutriation method (Boisseau 1957). The remaining material was checked Diet for larger and heavier organisms. Separated organisms of psam- mon and phyton were then assorted to relevant taxonomic units, The gut contents of the 704 fish (19.2 Lt 54.0) which were counted. contained 26 taxa (Table I). The other material found in The quantitative importance of different prey in the diet was the gut, consisting mostly of sand and bits of shell, expressed as follows: occurence frequency in percentage ( % F) , rarely of algae and foraminifera, was considered unin- number in percentage ( % N), mass in percentage ( % W) (Hynes tentional intake. The most frequently occurring prey 1950, Berg 1979). The subjective point methods (Hynes 1950, items ( % F > 30%) were Copepoda, Gastropoda, Bival- modified by Joyeux et al. 1991) was also applied as an alterna- via, Polychaeta and Ostracoda (Table I). All other taxa tive method for the determination of amount of the food items in appeared in less than 15% of the analysed guts. T h e terms of matter. Results of percentage mass were compared with most numerous food were Copepoda, followed by Gas- point percentages (%P). Points were given to preys after Joyeux tropoda, Bivalvia and Ostracoda, together constituting et al. (1991), Pampoulie & Bouchereau (1996) and Bouchereau almost 3/4 of all specimens in the prey (Table I). T h e & Guelorget (1999). Points for taxa not present in listed papers results based on percentage mass ( % W) were quite dif- were assigned from listed taxa of similar size. The main food ferent from results of percentage number ( % N) and per- index (IM F), modified with wet mass, was calculated to combine centage frequency ( % F). Gravimetrically, Polychaeta, the three methods used ( % F, % N, % W) (Kova i 2001). Feed- ing intensity was investigated using fullness index (IF) (Hureau Table I. – Diet spectrum of G. vittatus and quantitative contribu- 1970). Seasonal changes and ontogenetic shift in the diet tion of items; % F: occurrence frequency; % N: relative number; %W: relative biomass; % P: points in percentage; IM F: main food breadth were calculated using Levin’s standardised index (Bi) index. (Krebs 1989). The index range from 0 to 1; low values indicate diets dominated by few prey items and high values indicate gen- eralist diets (Krebs 1989). Measure of niche overlap was used to describe overlap between diet and prey offer in surrounding microhabitats (Zander & Hagemann 1987). The simplified

M o r i s i t a ’s index (Ci k) was calculated to compare overlap between ingested food and available food of four possible food sources (Krebs 1989). The overlap increases as the Morisita’s index increases from 0 to 1. Overlap is generally considered to be biologically significant when the value exceeds 0.60 (Xie e t a l . 2000). The 95% confidence limits of diet overlap and breadth were estimated using the jackknife method (Krebs 1989). Feeding selectivity (S e l) was evaluated according to Shorygin as mended by Berg (1979). Seasonal changes and ontogenetic shift in the diet composition were analysed by % N and % W and in feeding intensity by the fullness index (IF). A chi-square test was used to establish possible significant diff e r- ences in the diet composition by fish size and month. W h e n e v e r classes with expected frequencies less than five occurred, expected and observed frequencies for those classes were pooled with adjacent class to obtain large enough expected fre-

Vie Milieu, 2007, 57 (1/2) DIET OF GOBIUS VITTATUS IN THE ADRIATIC SEA 29

Fig. 2. – The feeding selectivity (S e l) of G . v i t t a t u s. Positive values mean preference, negative non-preference, -1 = total avoid- ance (meaning the component of the o ffered food was not preyed on by the fish).

Table II. – Top, ontogenetic shift and season- al changes in the fullness index of G. vittatus (IF, backtransformed mean and, in parenthe- ses, 95% confidence intervals of log-trans- formed data; Lt: total length in mm). Middle, analysis of variance of the fullness index of G. vittatus (IF) v e r s u s length classes (5 mm), seasons; and length classes and seasons; Wi = w i n t e r, Sp = spring, Su = s u m m e r, A u = autumn. Bottom, seasonal changes and ontogenetic shift in the diet breadth of G. vit - t a t u s ( L e v i n ’s standardised index Bi). Ranges in parentheses: 95% jackknife confidence intervals.

Paguridea and Gastropoda dominated, and made up over Feeding selectivity 1/2 of total prey biomass (Table I). The point method produced subjective distortion of most important food The overlap between ingested food and available food items in terms of matter (Polychaeta, Decapoda larvae, of all four possible food sources was biologically signifi- Isopoda and Gammaridae), compared to wet mass cant (simplified Morisita’s index, in parentheses 95% ( Table I). The calculation of the three estimated mea- confidence intervals): plankton 0.804 (0.794-0.816), pho- sures in main food index, IM F revealed three leading tophilic phyton 0.767 (0.746-0.789), scyaphilic phyton taxa: Polychaeta, Gastropoda and Bivalvia (Table I). 0.749 (0.726-0.772) and psammon 0.700 (0.671-0.729). Wide diet range, comprising 26 higher taxa, was con- Therefore, feeding selectivity was checked on all four firmed quantitatively with low IM F values for all prey possible food sources. The selectivity index showed the i t e m s . preference or non-preference of distinct components

Vie Milieu, 2006, 57 (1/2) 30 M. KOVA I

g r e w, and they became the dominant preys with Copepoda. Copepoda and Ostracoda were the most numerous preys during autumn as well, except in November, when a high number of Gammaridae was found in the diet (Fig. 3). G r a v i m e t r i c a l l y, the dominant preys in spring were Polychaeta, Gastropoda and Bivalvia. T h e gravimetric picture varied during summer and autumn. Polychaeta and Paguridea were almost constantly important in the diet in mass terms in these months. However, the diet was also influ- enced by one-month high biomass of some taxa (Leptostraca, Natantia, Mysidacea, Gammaridae, fishes), and it was biased by large specimens of some taxa (fishes, Leptostraca). Polychaeta, Gas- tropoda and Paguridea were the most important preys in mass terms in winter (Fig. 3). Diet breadth (Table II) was highest in the summer and lowest in winter and spring.

Ontogenetic shifts

No significant difference in the fullness index

(IF) was found among the size classes (Table II). Within each size class, percentage mass (%W) and percentage number (%N) were quite different (Fig. 4). The comparison of size classes showed highly significant differences (chi-square test, Fig. 3. – Monthly numerical ( % N) (a) and gravimetrical (%W) (b) com- d . f . = 5, P < 0.001) in number of all major preys position of diet of G. vittatus from April 2001 to March 2002. in the diet (Fig. 4), except Halacaridae (chi- square test, d.f. = 6, P > 0.05). The most numer- among the prey offer (Fig. 2). Copepoda and Polychaeta ous prey of young G. vittatus was Ostracoda, followed by were non-preferred, and Nematodes, Chaetognatha and Copepoda and Isopoda (Fig. 4). The importance of Ostra- Amphioxus were completely avoided. G. vittatus s h o w e d coda decreased with growth, and numerically the domi- preference to almost all other taxa in the diet, mostly dif- nant preys of medium size classes became Copepoda and ferent taxa of molluscs and crustaceans (Fig. 2). Bivalvia. Large fish, besides these two prey groups, ate numerous Gastropoda (Fig. 4). Although percentage mass Seasonal variation ( %W) of the various taxa varied with size (Fig. 4), there were significant differences only among Bivalvia, The intensity of feeding for G. vittatus varied season- Paguridea, fishes (chi-square test, d.f. = 3, P < 0 . 0 0 1 ) , ally (Table II). The fullness index (IF) was significantly Ostracoda (chi-square test, d.f. = 1, P < 0.01) and Cope- lower in autumn, compared with the other three seasons poda (chi-square test, d.f. = 1, P < 0.05). Gravimetrically, (Table II). Percentage mass (%W) and percentage number Mysidacea and Ostracoda dominated in the diet of the (%N) were rather different within each month (Fig. 3) and youngest G. vittatus. In medium size classes the most the diet composition of both indices varied over the important preys in terms of mass were Polychaeta, months (Fig. 3). Highly significant seasonal diff e r e n c e s Bivalvia and Gastropoda, while Polychaeta and Paguri- (chi-square test, d.f. = 11, P < 0.001) were found in dae were dominant for the largest fish (Fig. 4). T h e biomass and number of all major preys in the diet (Fig. 3), breadth of diet increased with fish size (Table II). except in number of polychaetes (chi-square test, d . f . = 11, P < 0.01) and mass of bivalves (chi-square test, d . f . = 11, P < 0.05). The mass of gammarids in the diet DISCUSSION was the only variable that did not fluctuate significantly over the months (chi-square test, d.f. = 11, P > 0 . 0 5 ) . The striped goby is a carnivore, as are most gobies N u m e r i c a l l y, the dominant preys in winter and spring (Miller 1986). Qualitative (26 taxa found in the diet) and were Copepoda, Gastropoda and Bivalvia. During sum- quantitative (IM F for all the taxa less than 25) data in this m e r, the number of Ostracoda and Isopoda in the diet research proved that this species is a generalist. Most

Vie Milieu, 2007, 57 (1/2) DIET OF GOBIUS VITTATUS IN THE ADRIATIC SEA 31

Valenciennes and G o b i u s cf. x a n t h o c e p h a l u s Heymer & Zander (wrongly as Gobius auratus Risso) (Zander & Hagemann 1989, Zander & Heymer 1992) and Gobius paganellus L i n n a e u s (Dunne 1978). The feeding selectivity, seasonal and ontoge- netic diet shift of the striped goby were unknown before the present research. No dominant food source for the striped goby was found among four distinct possible food sources and overlap of the gut content was biologically significant with all these microhabitats. The striped goby showed no feeding selectivity based on size or behaviour of the taxa, except for the avoidance of some infaunal taxa (nematods, sipunculids and amphioxus). Meiofauna and macrofauna were present among preferred as well as non-preferred p r e y. The striped goby therefore has unspecial- ized feeding habits. Low feeding intensity in autumn, during the period of decrease of the sea water temperature, as well as for the striped goby, has also been noticed for other gobiid species (Collins 1981, 1982, Joyeux et al. 1991, Kova i 2001). For all these species this is also the postspawning period (Miller 1986, Kova i 2001), when the gonado- somatic index reaches its lowest value for both sexes and specimens are in spent or recovering Fig. 4. – Numerical (%N) (a) and gravimetrical (%W) (b) composition of spent stages (Collins 1981, 1982, Kova i 2001). diet according to Lt (5 mm size classes) of G. vittatus. D i fferent results on seasonal changes of feeding intensity in temperate gobies were found by gobiid species generally consume crustaceans, poly- Vesey & Langford (1985) and Laffaille et al. chaetes and molluscs (Grossman et al. 1980), as was (1999). Hamerlynck & Cattrijsse (1994) noticed variation shown in this study for the striped goby as well. The only in the diet breadth for P. minutus and P. lozanoi similar to published data on the diet of striped goby were provided the present results on the striped goby, with high values in by Heymer & Zander (1978). Their results on diet were summer and autumn, and low values in spring. T h e restricted to frequency and abundance analyses of food reduced diet spectrum of the striped goby in winter and for 17 specimens from Banyuls (France) collected during spring resulted from high numerical dominance (> 7 0 % ) two summer months. Among the four leading taxa of prey of polychaetes, gastropods and bivalves in the diet. T h e of the present research, only polychaetes and copepods level of predation on polychaetes, gastropods (%W) and were present in food from Banyuls (France). On the other copepods (%N) was important throughout the year, while hand, sessile organisms, important in food at Banyuls- high consumption of other taxa was markedly seasonal. were not present (sponges) or were insignificant (algae) Constant increase of diet breadth along with the growth of in the present research. G. vittatus from the Kvarner area the striped goby was the result of increased capability of was predator and picker. The diet composition showed no l a rger fish to use food at different size. Percentage mass grazing activities as it was described in Heymer & Zander ( %W) and percentage number (%N) showed a quite dif- (1978). The striped goby in the present work mostly fed ferent pattern of diet composition during growth. Larg e on benthic organisms, but it also hunted free water hyper- d i fferences in diet composition among size classes have benthic fauna like mysids. Considerable differences in also been found for other European marine gobies: G . diet composition of the striped goby between Heymer & c o b i t i s (Gibson 1970), G. geniporu s (Zander & Heymer Zander (1978) and the present study showed limited sig- 1992), G. paganellus (Mazé 2004), G. ro u l e i ( K o v a i nificance of single research for the knowledge on general 2001), P. minutus and P. lozanoi (Hamerlynck & Cattri- feeding habits and preferences of the species. Large dif- jsse 1994). Numerically, meiofauna was more important ferences in diet composition among samples have also among most of the size classes, while macrofauna pre- been found within the same species for Gobius cobitis vailed gravimetrically among most of the size classes in Pallas (Gibson 1968, 1970, 1972), Gobius geniporu s this research. However, the trend related to growth was

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